The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2000-155358 filed May 25, 2000, which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an optical pickup apparatus enabling to read two or more kinds of recording medium, which are different in read wavelength, such as a compatible optical pickup apparatus for DVD/CD and the like. Particularly, the present invention relates to an optical pickup apparatus using a semiconductor laser element formed of a one-chip laser diode emitting two laser beams, which are different in wavelength from each other.
2. Description of the Related Art
Conventionally, a DVD/CD compatible reproducing apparatus sharing an optical pickup for a CD reproducing apparatus and a DVD reproducing apparatus has actively been proposed, and there are a DVD/CD compatible reproducing apparatus using a one-wavelength/bifocal optical pickup and a DVD/CD compatible reproducing apparatus using a two-wavelengths/bifocal optical pickup.
When a CD is compared with a DVD, a thickness of a protective layer of the DVD is about one-half (0.6 mm) that of a protective layer of the CD. Thus, in case of reproducing both of optical disks using a unifocal optical pickup, when a light beam is gathered so as to achieve optimization to an information record surface of the DVD, the protective layer of the CD, through which the light beam passes, is thicker than that of the DVD, therefore, aberration such as spherical aberration and the like occurs in the light beam so that the light beam cannot be gathered optimally to an information record surface of the CD. Also, since the CD is different from the DVD in a size of information pits formed for recording, it is necessary to form a beam spot with optimum size to the respective information pits on the information record surface of the CD or the DVD in order to exactly read the respective information pits.
Also, a size of the beam spot is proportional to a ratio of a wavelength of the light beam to a numerical aperture of an objective lens for gathering the light beam to the information record surface. That is, when it is assumed that a wavelength of the light beam is constant, the beam spot becomes smaller as the numerical aperture becomes larger. Accordingly, in the case of reproducing the CD and the DVD by the unifocal optical pickup, when it is constructed so that a wavelength of the light beam is constant and the numerical aperture is adapted for, for example, the information pits of the DVD, the beam spot becomes too small with respect to the information pits of the CD and distortion occurs in a reproduction signal on reproducing the CD and an exact reading becomes difficult. Hence, a DVD/CD compatible reproducing apparatus using a bifocal optical pickup enabling to focus at different positions on the same straight line and applying two laser beams for forming a beam spot with a proper size in correspondence with a size of each information pit has become the mainstream.
For example, an optical pickup apparatus shown in FIG. 13 is a DVD/CD compatible reproducing apparatus in which an optical path of a light beam emitted from a first light source 10 for CD and an optical path of a light beam emitted from a second light source 15 for DVD are mixed by a first beam splitter 13 acting as a prism to apply any one of two light beams emitted from the two light source, respectively, to a bifocal lens comprising an objective lens and a diffraction element, and a configuration and operation will be described briefly.
In FIG. 13, the first light source 10 generates a laser beam (shown by a broken line) with a wavelength (780 nm) most suitable for an information reading from a CD according to a driving signal from a first driving circuit 11, and the laser beam is applied to the first beam splitter 13 through a grating 12 for generating three beams. The first beam splitter 13 reflects the laser beam emitted from the first light source 10 and guides the reflected light to a second beam splitter 14.
On the other hand, the second light source 15 placed at a 90xc2x0 with respect to the first light source 10 generates a laser beam (shown by a solid line) with a wavelength (650 nm) most suitable for an information reading from a DVD according to a driving signal from a second driving circuit 16, and the laser beam is applied to the first beam splitter 13 through a grating 17. The first beam splitter 13 transmits the laser beam emitted from the second light source 15 and guides the laser beam to the second beam splitter 14.
The second beam splitter 14 guides the laser beam supplied through the first beam splitter 13, namely the laser beam emitted from the first light source 10 or the second light source 15 to a bifocal lens 19 through a collimator lens 18. The bifocal lens 19 gathers the laser beam from the second beam splitter 14 at one point to be information reading light beam and the information reading light beam is applied to an information record surface of an optical disk 21 rotated and driven by a spindle motor 20.
The laser beam emitted from the first light source 10 (shown by a broken line) is gathered by the bifocal lens 19 to focus on an information record surface C of the optical disk 21. Also, the laser beam emitted from the second light source 15 (shown by a solid line) is gathered by the bifocal lens 19 to focus on an information record surface D of the optical disk 21.
The reflected light occurring by applying the information reading light beam from the bifocal lens 19 to the optical disk 21 passes through the bifocal lens 19 and the collimator lens 18, is reflected at the second beam splitter 14, passes through a cylindrical lens 22, which is an astigmatism generation element, and is applied to a photodetector 23. The photodetector 23 generates a signal having a level corresponding to light intensity of the applied light and supplies the signal to an information data reproducing circuit 24 and a disk determination circuit 25 as a reading signal.
The information data reproducing circuit 24 generates a digital signal based on the obtained reading signal and further performs demodulation and error correction to the digital signal to reproduce information data. The disk determination circuit 25 identifies a kind of the optical disk 21 based on a size of a beam spot formed at the time of applying a laser beam to the optical disk 21 and supplies the kind to a controller 26 as disclosed in, for example, Japanese Unexamined Patent Application No. Hei. 10-255274 by the present applicant. According to a disk identification signal, the controller 26 drives and controls any one of the first driving circuit 11 and the second driving circuit 16 selectively in a drive state. The controller 26 drives only the first driving circuit 11 when a disk identification signal indicating a CD is obtained from the disk determination circuit 25. Therefore, the laser beam emitted from the first light source 10 is applied to the optical disk 21 through an optical system comprising the grating 12, the first beam splitter 13, the second beam splitter 14, the collimator lens 18, and the bifocal lens 19. Then, the reflected light (return light) reflected at the information record surface of the optical disk 21 passes through the bifocal lens 19 and the collimator lens 18, and is reflected at the second beam splitter 14, and passes through the cylindrical lens 22, and is applied to the photodetector 23.
Also, the controller 26 drives only the second driving circuit 16 when a disk identification signal indicating a DVD is obtained from the disk determination circuit 25. Therefore, the laser beam emitted from the second light source 15 is applied to the optical disk 21 through an optical system comprising the grating 17, the first beam splitter 13, the second beam splitter 14, the collimator lens 18 and the bifocal lens 19. That is, it is constructed so that the first light source 10 for generating a laser beam having a wavelength most suitable for an information reading from the optical disk 21 with a relatively low recording density as the CD and the second light source 15 for generating a laser beam having a wavelength most suitable for an information reading from the optical disk 21 with a high recording density as the DVD are provided and the light source corresponding to the kind of the optical disk 21 targeted for reproduction is alternatively selected. Then, the reflected light (return light) reflected at the information record surface of the optical disk 21 passes through the bifocal lens 19 and the collimator lens 18, and is reflected at the second beam splitter 14, and passes through the cylindrical lens 22, and is applied to the photodetector 23.
As described above, the DVD/CD compatible reproducing apparatus requiring two light sources requires the prism to increase a cost compared with an optical pickup apparatus having one light source. When the first light source 10 is applied from one side of the first beam splitter 13, it is necessary to apply the second light source 15 from the other side perpendicular to the first light source 10 and thus, there is a problem that space for placing an optical system widen and the optical pickup apparatus becomes large.
The invention is implemented in view of the problem, and an object of the invention is to provide an optical pickup apparatus in correspondence with two wavelengths enabling to do miniaturization without using a prism.
In order to solve the above problem, an optical pickup apparatus according to a first aspect of the invention enabling to read information of a plurality of recording mediums having different reading wavelengths from each other, the optical pickup apparatus comprises:
a light emission unit including a first light emission source adapted to emit a first laser beam, and a second light emission source disposed adjacent to the first light emission source and adapted to emit a second laser beam whose wavelength is different from that of the first laser beam, and
an photodetection unit; and
a beam splitter having first and second half mirrors,
wherein the beam splitter guides the first and second laser beams toward the recording medium and guides a reflected beam reflected at the recording medium toward the photodetection unit.
In a second aspect of the invention, the optical pickup apparatus according to the first aspect of the invention is provided wherein the first half mirror is disposed with respect to the second half mirrors so that:
the first laser beam emitted from the light emission unit is reflected at the first half mirror to be guided toward the recording medium; and
the second laser beams emitted from the light emission unit passes the first half mirror, is reflected at the second half mirror, and passes through the first half mirror to be guided toward the recording medium.
An optical pickup apparatus according to a third aspect of the invention is the optical pickup apparatus according to any one of the first and second aspects of the invention wherein the first and second laser beam between the beam splitter and the recording medium have the same optical path.
An optical pickup apparatus according to a fourth aspect of the invention is the optical pickup apparatus according to the first aspect of the invention wherein the first and second half mirrors are inclined with respect to the first and second laser beams emitted from the light emission unit.
An optical pickup apparatus according to a fifth aspect of the invention is characterized in that in the optical pickup apparatus according to any one of the first to third aspects of the invention, wherein the first and second half mirrors are substantially parallel to each other.
An optical pickup apparatus according to a sixth aspect of the invention is characterized in that in the optical pickup apparatus according to any one of the first to fourth aspects of the invention, the first and second laser beams reflected at the recording medium passes through the first half mirror and passes through the second half mirror to be guided toward the photodetection unit.
An optical pickup apparatus according to a seventh aspect of the invention is characterized in that in the optical pickup apparatus according to the first to fifth aspects of the invention, a parallel flat plate adapted to provide astigmatism to the first and second laser beams reflected at the recording medium is fixed to the second half mirror.